Photo-induced trimming of chalcogenide-assisted silicon
photonic circuits
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Citation
Melloni, Andrea, Stefano Grillanda, Antonio Canciamilla, Carlo
Ferrari, Francesco Morichetti, Michael Strain, Marc Sorel, Vivek
Singh, Anu Agarwal, and Lionel C. Kimerling. “Photo-induced
trimming of chalcogenide-assisted silicon photonic circuits.” In
Silicon Photonics VII, edited by Joel Kubby and Graham T.
Reed, 82660A-82660A-8. SPIE - International Society for Optical
Engineering, 2012. SPIE © 2012
As Published
http://dx.doi.org/10.1117/12.908521
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SPIE
Version
Final published version
Accessed
Thu May 26 09:01:40 EDT 2016
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http://hdl.handle.net/1721.1/81199
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Invited Paper
Photo-induced trimming of chalcogenide-assisted silicon photonic
circuits
Andrea Melloni*a, Stefano Grillandaa, Antonio Canciamillaa, Carlo Ferraria, Francesco Morichettia,
Michael Strainb, Marc Sorelb, Vivek Singhc, Anu Agarwalc, Lionel C. Kimerlingc
a
Dipartimento di Elettronica e Informazione, Politecnico di Milano, 20133 Milano, Italy;
b
School of Engineering, University of Glasgow, Glasgow G12 8LT, United Kingdom;
c
Microphotonics Center, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139,
USA
ABSTRACT
We present an innovative and efficient technique for post-fabrication trimming of silicon photonic integrated circuits
(PICs). Our approach exploits the high photosensitivity of chalcogenide glasses (ChGs) to induce local and permanent
modifications of the optical properties and spectral responses of ChG-assisted silicon devices. We experimentally
demonstrate the potential of this technique on ring resonator filters realized on a silicon-on-insulator platform, for which
post-fabrication treatments enable to counteract the strong sensitivity to technological tolerances. Photosensitive ChGassisted silicon waveguides were realized by deposition of a As2S3 chalcogenide layer on top of conventional silicon
channel waveguides. A resonant wavelength shift of 6.7 nm was achieved, largely exceeding the random resonance
spread due to fabrication tolerances. Neither the ChG layer deposition, nor the trimming process introduces appreciable
additional losses with respect to the bare silicon core waveguide. Performances of the trimming technique, such as speed
and saturation effects, as well as nonlinear behavior and infrared writing issues are investigated and experimentally
characterized.
Keywords: Chalcogenide-assisted photonics, silicon photonics, visible light trimming, photo-induced trimming,
photosensitivity, programmable photonics, reconfigurable photonics
1. INTRODUCTION
Silicon-on-insulator (SOI) is one of the most promising technological platform for the realization of photonic integrated
circuits (PICs) for all-optical processing, network and sensing applications1-6. Thanks to the high confinement of the light
in the core of waveguides, SOI technology allows the fabrication and exploitation of devices with large bandwidth, high
selectivity and ultra-small footprint.
However, one of the main impairments to the full exploitation of SOI devices is the strong sensitivity to fabrication
tolerances. This limitation, common to all high-index contrast technologies, is particularly relevant in the SOI platform
where, as a rule of thumb, a deviation of 1 nm in the width of the waveguide results in a wavelength shift of 1 nm in the
spectral response of any interferometric device. This suggests that, even in the most advanced and reliable fabrication
processes, devices with bandwidth of few tens of GHz are hardly controllable7.
Therefore, for the complete utilization and exploitation of the properties of the SOI technology, it is mandatory to
provide post-fabrication treatments to compensate for technological tolerances and to recover desired functionalities and
specifications.
In the past several approaches have been proposed. One of the most common solution is to provide on-chip active tuning
techniques, like local heating through thermo-optic actuators6 or carrier injection through p-i-n junctions8. These
methods however are “power hungry” and require “always-on” mechanisms. Also, these approaches are efficient when a
fast and continuous reconfiguration is required, but are less effective to counteract fabrication tolerances.
*melloni@elet.polimi.it; phone +39 02 2399 3546; fax +39 02 2399 3413;
Silicon Photonics VII, edited by Joel Kubby, Graham Trevor Reed, Proc. of SPIE Vol. 8266,
82660A · © 2012 SPIE · CCC code: 0277-786X/12/$18 · doi: 10.1117/12.908521
Proc. of SPIE Vol. 8266 82660A-1
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On the other side, different techniques for the trimming of SOI waveguides have been presented, such as oxide
compaction induced by electron beam exposure9, or oxidation by the tip of an atomic force microscope10. These solutions
however are quite expensive, require sophisticated equipments, and are hardly in line with low-cost processes.
Other approaches exploit waveguides with UV sensitive polymer cladding, whose properties are tailored by UV
irradiation. However, polymers suffer from poor time stability and optical degradation at relatively low temperatures11.
In this contribution we propose a new, innovative technique to compensate for fabrication tolerances of SOI devices, and
therefore to recover functionalities and fully exploit their potentialities. Our approach employs chalcogenide-assisted
silicon waveguides, where the silicon core is covered with a thin film of As2S3 chalcogenide glass. The photosensitivity
of ChGs is therefore utilized to tailor the local optical properties of the silicon core waveguide. Our trimming technique
is simple, accurate, low-cost and low-power consuming. First, we introduce the chalcogenide-assisted silicon waveguide,
describing its properties and the central role of ChGs. Then, we describe the main characteristics and performances of the
technique employed to trim the devices and issues related to nonlinear behavior and infrared writing.
2. CHALCOGENIDE-ASSISTED SILICON WAVEGUIDES: FABRICATION AND
CHARACTERISTICS
ChGs are a class of amorphous semiconductor compounds that have been widely employed in optical applications to
realize mainly optical fibers and thin film devices. Recently, thanks to the high transparency to infrared radiation and to
the high photosensitivity to near band-gap visible light12, they are emerging as a promising technological platform to
realize integrated optical devices13. In fact, they have been employed to directly write planar waveguides14 and a wide
variety of devices, ranging from Bragg gratings15 to photonic crystal cavities16, ring resonators17 and, more recently, to
modify the optical properties of complex coupled resonator filters18,19.
The ability of locally inducing a permanent refractive index change in the chalcogenide material by means of a lowintensity, visible light illumination is the key to operate a post fabrication trimming functionality. In fact by changing the
refractive index of the material it is possible to change the effective refractive index of the waveguide, and therefore to
finely tailor its optical properties.
In this work ChGs are employed as a cladding material to cover silicon core waveguides, thus giving birth to
chalcogenide-assisted silicon waveguides (Fig. 1). Silicon channel waveguides with a width of 500 nm were written on a
SOI wafer with a 220 nm thick silicon layer by means of electron-beam lithography and inductively coupled plasma
reactive ion etching process20. Waveguide width is widened up to 5 µm approaching the chip end-facets in order to
improve the fiber-to-waveguide coupling efficiency. The oxide buffer layer has a thickness of 2 µm. As depicted in Fig.
1(a), a layer of As2S3 chalcogenide glass with a thickness of 420 nm was thermally evaporated21 on top of the silicon core
waveguide. The As2S3 upper cladding does not introduce further appreciable propagation losses, which are about 2
dB/cm. Fig. 1(b) reports a scanning electron microscope (SEM) image of the waveguide cross section, showing a good
uniformity and conformity of the deposition process. The 70 nm thick layer between the silicon core and the As2S3 upper
cladding is the hydrogen silsesquioxane (HSQ) electron-beam resist, which has not been removed since it does not affect
the waveguide properties.
The effect of the ChG cladding and the impact of the photosensitivity of the material on the optical properties of the
waveguide have been investigated through electromagnetic simulations in order to optimize the thickness of the As2S3
glass. A variation in the index of the As2S3 coverage results in a variation of the effective index of the waveguide and
therefore in a wavelength shift of any interferometric device. Fig. 2 reports simulations of the effective index change (a)
and of the wavelength shift (b) versus the ChG thickness for both TE (blue solid line) and TM (red dashed line)
polarizations, given a variation of 4·10-2 in the index of the As2S3 glass. The green dotted line indicates the value of ChG
thickness chosen for the fabrication of the devices. As shown in the figures, the effective index change is always greater
on TM polarization than on TE polarization because TM mode is less confined than TE mode. Therefore the same
amount of refractive index change corresponds to a higher effective index change on TM polarization than on TE
polarization.
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Figure 1. (a) Schematic view and (b) SEM image of the cross section of the waveguide fabricated for the trimming
experiments.
Figure 2. Electromagnetic simulations of the effective index change (a) and of the wavelength shift (b) versus the
chalcogenide thickness for a variation of 4·10-2 in the index of the material for both TE (blue solid line) and TM
(red dashed line) polarization. The green dotted line refers to the value of ChG thickness employed for the
fabrication of the devices.
3. EXPERIMENTAL TRIMMING: TECHNIQUE, PROPERTIES AND PERFORMANCES
The trimming functionality of the chalcogenide-assisted silicon waveguide was demonstrated on a ring resonator coupled
to a bus waveguide in an all-pass filter configuration as shown in the optical microscope image reported in Fig. 3. The
ring has a circular shape with radius of 40 µm and a free-spectral-range (FSR) of 2.48 nm (310 GHz). Both the ring and
the waveguide are covered with As2S3 glass.
In order to operate the trimming of the device, a multimode optical fiber directly coupled to a halogen lamp was
positioned on top of the ring resonator. The position of the fiber (which has a mode field diameter of 30 µm) was
controlled with a micropositioning stage in order to locally and selectively expose specific areas of the device. The lamp
employed in the experiment has a spectrum in the visible region (i.e. wavelength ranging from 450 nm to 650 nm) with
an intensity that can be varied from 0.5 mW/cm2 to 10 mW/cm2.
Fig. 4(a) reports the spectral response of the ring resonator (black dashed line) and the shift of the resonant frequency
over a whole free-spectral-range (from blue to red solid line) when exposed to visible light with an intensity of 1
mW/cm2. The trimming process does not introduce any additional loss to the waveguide, in fact no significant changes
are visible in the spectral response, shape and width of the notches after the procedure.
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Figure 3. Top view image of the ring resonator filter fabricated for the visible light trimming experiment.
Fig. 4(b) shows the behavior of resonant wavelength shift versus the time of exposure to visible light. Visible light
illumination with an intensity of 1 mW/cm2 was maintained over time in order to investigate the saturation limit of the
trimming process. A wavelength shift of about 6.7 nm was obtained with a time constant (90%) of 400 min (Fig. 4(b)).
The wavelength shift achieved corresponds to a variation in the effective index of the waveguide of 2·10-2. After 90 days
of storage in dark conditions only a little deviation of 0.15 nm was observed, corresponding to 2% of the total
wavelength shift induced.
Figure 4. (a) Spectral response of the ring resonator filter (black dashed line) and resonant frequency shift over a full
free-spectral-range during the visible light exposure (from blue to red solid line); (b) Wavelength shift of the ring
resonance versus time: complete saturation curve for light intensity of 1 mW/cm2; (c) Wavelength shift of the ring
resonance versus time: comparison of the trimming velocity for visible light with various intensities (from 1
mW/cm2 to 3 mW/cm2).
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The trimming performance was observed also for different light intensities: Fig. 4(c) reports the wavelength shift
measured for 1 mW/cm2 (black circles), 2 mW/cm2 (blue squares) and 3 mW/cm2 (green triangles). It can be noticed that
the speed of the trimming process is linear with the intensity of light, with a figure of 60IT pm/min (where IT is the
intensity of the trimming light in mW/cm2 units). The shift over the entire FSR is obtained in 38 min with an intensity of
1 mW/cm2, thus indicating that the entire process can be carried out in 1 min at the low intensity of 40 mW/cm2.
The design of the waveguide can be further optimized to improve the performances of the trimming functionality. For
instance, by narrowing the width of the waveguide it is possible to strengthen the interaction between the ChG upper
cladding and the light propagating in the silicon core, and thus to achieve a better trimming performance. However, it
must be noticed that a narrower waveguide produces a higher level of losses and backscattering22. Therefore a trade-off
between the device specifications and the “trimmability” of the waveguide must be found.
4. NONLINEAR BEHAVIOR AND IR-WRITING
It is well known that, when the propagating optical power increases, TPA (two photon absorption) and FCD (free carrier
dispersion) effects arise in SOI platform. These non linear phenomena are responsible for both propagation losses
increase and refractive index change, that are power dependent and can significantly affect device response and
performance7.
We recently demonstrated that ChG-based PICs exhibit a certain degree of photosensitivity also to IR radiation (1550 nm
wavelength), when optical power density in the waveguide exceeds the threshold of 0.1 GW/cm2. For these reasons, we
investigated the non linear behavior of the proposed ChG assisted SOI waveguides.
The spectral response of a ChG-assisted SOI ring resonator was measured with a pump-probe experimental setup (Fig. 5)
with the launch optical pump power that increases from -1 dBm to 17 dBm (solid lines, red arrow) and then decreases
down to -1 dBm again (dotted lines, blue arrow).
Due to the build-up factor of the resonator, that is about 16, the pump power propagating inside of the cavity is
correspondingly increased up to about 250 mW (taking into account also coupling losses), enhancing therefore all the
non linear effects.
Figure 5. Spectral response of the ring resonator measured with a pump-probe experimental setup with the launch
optical pump power that increases from -1 dBm to 17 dBm (solid lines, red arrow) and then decreases down to -1
dBm (dotted lines, blue arrow).
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The resonance wavelength shift of the ring’s notch is reported in Fig. 6 versus the rising (red line) and decreasing (blue
line) value of the launch optical pump power.
As expected, two effects can be observed:
- the FCD-induced refractive index change in the silicon core produces a maximum non linear red shift of about 30 pm;
this effect is completely reversible and disappears when the pump power decreases down to the linear regime. This
behavior is in good agreement with previous results on SOI ring resonators7, demonstrating that the ChG cladding does
not remarkably modify the TPA and FCD induced effects on silicon waveguides.
- the IR-photosensitivity of the ChG cladding produces a red shift of about 11 pm: this effect is the result of an exposure
of about 20 minutes to a 1550 nm wavelength radiation with power above 10 dBm. This effect is permanent and the
wavelength shift is maintained also when the optical pump power is decreased down to the linear regime.
Figure 6. Resonance wavelength shift of the ring’s notch versus the rising (red line) and decreasing (blue line) value of
the launch optical pump power.
5. CONCLUSIONS
In this contribution we have presented a technique to operate a post-fabrication trimming of PICs and have demonstrated
its effectiveness on ChG-assisted silicon waveguides.
Our approach exploits the high photosensitivity of ChGs to visible light to induce a change in the optical properties of
the silicon device. By selectively exposing the ChG cladding to visible light it is possible to change its refractive index
and therefore to induce a variation in the effective index of the silicon core waveguide.
With respect to different approaches, our technique is simple, low-cost and low-power consuming, and requires only a
common halogen lamp with no need of expensive equipment or additional fabrication steps.
The effective index change achieved, which exceeds 10-2, is large enough to compensate the fabrication tolerances of the
typical SOI platform. It enables to target desired specifications and recover the desired spectral response with no need of
tight fabrication tolerances or power-hungry always-on actuators. In fact the functionality of the circuit is held after
trimming with no continuous power consumption.
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Moreover, this powerful approach offers promising applications in the realization of reconfigurable and programmable
optical circuits, where specific and customized functionalities can be written and erased on generic architectures.
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